1. Field of the Invention
The present invention relates to an entering detecting system for vehicles, and in particular to an intruding detecting system for vehicles suitable for detecting intruding of an unauthorized person by using a Doppler shift of an ultrasonic wave which occurs when a person enters a vehicle compartment.
2. Related Art
A conventional entering detecting system for vehicles of this kind is disclosed in U.S. Pat. No. 5,856,778 (JP-A-9-274080).
In this entering detecting system, an ultrasonic wave transmitter installed in a vehicle compartment transmits an ultrasonic wave into the vehicle compartment in response to an oscillation pulse of an oscillator circuit, and an ultrasonic receiver installed in the vehicle compartment generates a reception signal upon receiving the ultrasonic wave reflected by a person or the like in the vehicle compartment.
The reception signal is amplified by an amplifier circuit, and is output as a Doppler shift signal through a waveform shaper circuit, a phase difference output circuit (exclusive-OR gate) and a band pass filter. The reception signal is envelope-detected by an envelope detector circuit and is output as an envelope detector signal. Entering of person is detected from the Doppler shift signal after checking the level of the envelope detection signal, that is, after checking that the level of the reception signal is sufficiently large.
In the above entering detecting system, the level of the reception signal varies with variation in convection of air in the vehicle compartment. Specifically, the level of the reception signal abruptly varies due to abrupt variations in the interference of the ultrasonic wave, if the convection condition of air abruptly varies due to variations in temperature in the vehicle compartment or in the flow of wind entering from outside. Therefore, it is still likely that the level of the reception signal rises only temporarily because of the abrupt variation in the convection of air, while the level of the reception signal is normally low. For this reason, it is hard to detect entering of person with high accuracy, unless the period of continuation of the reception signal is monitored.
Further, if the amplifier circuit of the entering detecting system has an operational amplifier to amplify the reception signal by the operational amplifier, the phase of the amplified output of the operational amplifier greatly varies when a transistor such as an FET which is at the output stage of the operational amplifier saturates. The variation in the phase is output as the Doppler shift signal through the waveform shaper circuit, the phase difference output circuit and the band pass filter. As a result, entering of person may be detected erroneously even when no person actually enters the vehicle compartment.
It is assumed that the oscillator circuit generates the oscillation pulse (P in FIGS. 9 to 11) at 40 KHz and the waveform shaper circuit generates the waveform-shaped pulse (R in FIG. 9), under the condition that no person is entering the vehicle compartment. In this instance, the amplifier circuit generates the amplified output having a waveform indicated by A in FIG. 9 and the phase difference output circuit generates the phase difference output indicated by B in FIG. 9, unless the transistor such as an FET at the output stage of the amplifier circuit is not in saturation.
On the contrary, the operational amplifier generates the amplified output having the saturated waveform indicated by A1 in FIG. 10, the waveform shaper circuit generates the waveform-shaped pulse indicated by R1 in FIGS. 10 and 11 and the phase difference output circuit generates the phase difference output having the waveform indicated by B1 in FIG. 10, if the transistor such as an FET at the output stage of the operational amplifier is in saturation.
It is understood from this that each pulse width of the waveform-shaped pulse of the waveform shaper circuit and the phase difference output of the phase difference output circuit vary with the saturation of the transistor (arrow S in FIG. 11) and restoration from the saturation, that is, non-saturation (arrow S1 in FIG. 11). This means that the phase relation between the transmitted signal and the reception signal has changed. This phase change arises from the time required for the transistor such as an FET at the output stage of the operational amplifier to be restored from the saturation when it saturates.
As a result, a false Doppler shift signal is output through the band pass filter in the waveform indicated by D in FIG. 11, when the amplified output of the operational amplifier repeats saturation and non-saturation.
The phase difference output of the phase difference output circuit basically varies only when a reception signal frequency varies. However, the false Doppler shift signal is generated when the saturation and non-saturation of the amplified output of the operational amplifier occurs repeatedly, even if the reception signal frequency and the phase difference between the transmitted ultrasonic wave and the received ultrasonic wave do not vary. Thus, it is likely to occur that an occurrence of Doppler shift is caused by a person intruding into the vehicle compartment in response to the false Doppler shift signal.
Testing is conducted to study the relation between the Doppler shift signal (corresponding to a one cycle period of a Doppler shift) caused in response to the variation in temperature in the vehicle compartment when no one is entering the vehicle compartment and the level of the reception signal indicating the received ultrasonic wave. It is ascertained through this testing that the false Doppler shift signal was not generated if the level of the reception signal was high, while the false Doppler shift signal was generated when the level of the reception signal was high.
It is therefore possible to enhance the accuracy of detecting entering of person by basically determining that the Doppler shift signal is caused by entering of person when the level of the reception signal remains high for a sufficient period.
It also is ascertained that the transistor at the output stage of the operational amplifier does not saturate if the construction of associated circuits of the operational amplifier are improved.